1. Field of the Invention
The invention relates to a turbine engine having two unducted (or “open rotor”) contrarotating propellers, namely an upstream propeller and a downstream propeller.
2. Description of the Related Art
In conventional manner, such a turbine engine has two contrarotating propellers on a common axis, respectively an upstream propeller and a downstream propeller, which propellers extend radially outside the nacelle of the turbine engine. These two propellers are driven in rotation by a power turbine arranged at the outlet from a combustion chamber, and they are axially spaced apart from each other by some minimum distance in order to limit sound emissions.
In a known architecture, the contrarotating propellers are mounted axially downstream from the power turbine, which has a plurality of moving blades arranged axially in alternation with stationary vanes carried by a casing of the turbine engine. The rotor of the power turbine is driven in rotation by the combustion gas and it is connected to an inlet shaft of a step-down gearbox having two contrarotating outlet shafts, each driving a respective one of the upstream and downstream propellers in opposite directions. Such a step-down gearbox is described in document U.S. Pat. No. 4,997,414.
That configuration enables the power turbine to operate at a high speed for which its efficiency is at a maximum with a small number of stages, e.g. three stages. Furthermore, the contrarotating propellers are capable of rotating at a speed that is slower than the rotor of the power turbine, because of the incorporated step-down gearbox, thereby limiting sound emission from the propellers in operation.
Nevertheless, the minimum axial spacing that needs to be complied with between the contrarotating propellers, and the way they are arranged downstream from the power turbine impose an increase in axial length of the turbine engine, leading to the two contrarotating propellers being cantilevered out on the downstream portion of the turbine engine. This is conventionally compensated by reinforcing the structures for supporting the upstream and downstream propellers, thereby increasing the weight of the turbine engine and thus its fuel consumption.
In another type of architecture, the power turbine is mounted between the upstream and downstream propellers and has two contrarotating rotors, an inner rotor and an outer rotor, each having a plurality of rows of blades arranged axially in alternation with one another. The inner and outer rotors are connected directly to the upstream and downstream propellers, respectively.
Direct mechanical drive of the upstream and downstream propellers by the inner and outer rotors respectively of the power turbine prevents good efficiency being obtained in the power turbine while using only a small number of blade stages, as is possible in the above-described architecture using a step-down gearbox, given that the speeds of rotation of the upstream and downstream propellers must be sufficiently slow to avoid generating high levels of sound nuisance.
In order to use such a turbine engine, it is necessary to increase the number of stages in the power turbine up to eight or even ten stages, thereby complicating assembly of the turbine engine and increasing its cost.